Call a class method without instantiating the class

How can I convert my amount into words? I tried this code but it gave me this error when I call the function.

Fatal error: Call to undefined function Convert()

Did I do something wrong?

if (isset($_REQUEST["generate"])) {
    $amount = $_POST["amount"];
    $amountinword = Convert($amount);
}


Class Convert {
    var $words = array();
    var $places = array();
    var $amount_in_words;
    var $decimal;
    var $decimal_len;
    
    function Convert($amount, $currency="Pesos") {
        $this->assign();
        
        $temp = (string)$amount;
        $pos = strpos($temp,".");
        if ($pos) {
            $temp = substr($temp,0,$pos);
            $this->decimal = strstr((string)$amount,".");
            $this->decimal_len = strlen($this->decimal) - 2;
            $this->decimal = substr($this->decimal,1,$this->decimal_len+1);
        }
        $len = strlen($temp)-1;
        $ctr = 0;
        $arr = array();
        while ($len >= 0) {
            if ($len >= 2) {
                $arr[$ctr++] = substr($temp, $len-2, 3);
                $len -= 3;
            } else {
                $arr[$ctr++] = substr($temp,0,$len+1);
                $len = -1;
            }
        }
        
        $str = "";
        for ($i=count($arr)-1; $i>=0; $i--) {
            $figure = $arr[$i];
            $sub = array(); $temp="";
            for ($y=0; $y<strlen(trim($figure)); $y++) {
                $sub[$y] = substr($figure,$y,1);
            }
            $len = count($sub);
            if ($len==3) {
                if ($sub[0]!="0") {
                    $temp .= ((strlen($str)>0)?" ":"") . trim($this->words[$sub[0]]) . " Hundred";
                }
                $temp .= $this->processTen($sub[1], $sub[2]);
            } elseif ($len==2) {
                $temp .= $this->processTen($sub[0], $sub[1]);
            } else {
                $temp .= $words[$sub[0]];
            }
            if (strlen($temp)>0) {
                $str .= $temp . $this->places[$i];
            }
        }
        $str .= " " . $currency;
        if ($this->decimal_len>0) {
            $str .= " And " . $this->decimal . "/" . $this->denominator($this->decimal_len+1) .  " Cents";
        }
        $this->amount_in_words = $str;
    }
    
    function denominator($x) {
        $temp = "1";
        for ($i=1; $i<=$x; $i++) {
            $temp .= "0";
        }
        return $temp;
    }
    
    function display() {
        echo $this->amount_in_words;
    }

    function processTen($sub1, $sub2) {
        if ($sub1=="0") {
            if ($sub2=="0") {
                return "";
            } else {
                return $this->words[$sub2];
            }
        } elseif ($sub1!="1") {
            if ($sub2!="0") {
                return $this->words[$sub1."0"] . $this->words[$sub2];
            } else {
                return $this->words[$sub1 . $sub2];
            }
        } else {
            if ($sub2=="0") {
                return $this->words["10"];
            } else {
                return $this->words[$sub1 . $sub2];
            }
        }
    }

    function assign() {
        $this->words["1"] = " One";             $this->words["2"] = " Two";
        $this->words["3"] = " Three";       $this->words["4"] = " Four";
        $this->words["5"] = " Five";            $this->words["6"] = " Six";
        $this->words["7"] = " Seven";           $this->words["8"] = " Eight";
        $this->words["9"] = " Nine";
    
        $this->words["10"] = " Ten";            $this->words["11"] = " Eleven";
        $this->words["12"] = " Twelve";     $this->words["13"] = " Thirten";
        $this->words["14"] = " Fourten";        $this->words["15"] = " Fiften";
        $this->words["16"] = " Sixten";     $this->words["17"] = " Seventen";
        $this->words["18"] = " Eighten";        $this->words["19"] = " Nineten";

        $this->words["20"] = " Twenty";     $this->words["30"] = " Thirty";
        $this->words["40"] = " Forty";      $this->words["50"] = " Fifty";
        $this->words["60"] = " Sixty";      $this->words["70"] = " Seventy";
        $this->words["80"] = " Eighty";     $this->words["90"] = " Ninety";
    
        $this->places[0] = "";                  $this->places[1] = " Thousand";
        $this->places[2] = " Million";      $this->places[3] = " Billion";
        $this->places[4] = " Thrillion";
    }
}
    
?>

Problem

You aren't creating an instance of Convert so PHP thinks it's a function.

Solution:

$convert = new Convert($amount);
$convert->display();

On a side note, your Convert method is acting as a constructor function. This is a backwards compatibility function from older versions of PHP. You should follow best-practices by renaming it to __construct now. From the manual:

Warning: Old style constructors are DEPRECATED in PHP 7.0, and will be removed in a future version. You should always use __construct() in new code.

Working example.

Also on a side note - you allow for the currency to be provided as an argument to the constructor method, so you should also allow for the decimal point name as well (Cents) - it's currently hard coded into your Convert method.